Gradient design of imprinted anode for stable Zn-ion batteries

Achieving long-term stable zinc anodes at high currents/capacities remains a great challenge for practical rechargeable zinc-ion batteries. Herein, we report an imprinted gradient zinc electrode that integrates gradient conductivity and hydrophilicity for long-term dendrite-free zinc-ion batteries. The gradient design not only effectively prohibits side reactions between the electrolyte and the zinc anode, but also synergistically optimizes electric field distribution, zinc ion flux and local current density, which induces preferentially deposited zinc in the bottom of the microchannels and suppresses dendrite growth even under high current densities/capacities. As a result, the imprinted gradient zinc anode can be stably cycled for 200 h at a high current density/capacity of 10 mA cm−2/10 mAh cm−2, with a high cumulative capacity of 1000 mAh cm−2, which outperforms the none-gradient counterparts and bare zinc. The imprinted gradient design can be easily scaled up, and a high-performance large-area pouch cell (4*5 cm2) is also demonstrated.

Regarding novelty, the author's correctly mention in the introduction that 3D Zn anode designs and Sn-based current collector or anode coatings for Zn have been explored in other studies, and there are a few other papers recently published in the literature on both of these topics. The same can be said for PVDF coatings on Zn, such as the following report: (https://www.sciencedirect.com/science/article/pii/S1385894721001832). Thus, while the individual approaches are not novel, it is the combination of these approaches that is the unique feature of this manuscript and would be of some interest to the Zn community.
In general, the work supports the conclusions, the methodology is sound, and there is enough detail provided in the methods for the work to be reproduced. This paper is suitable for publication in Nature Communications if the following questions and/or minor revisions can be addressed: 1.) Are the anode images in Figure S11 shown in the charged (plated) or discharged (stripped) state? This should be included in the caption for context. 4.) Is the Sn surface treatment self-limiting in terms of thickness or was there some optimization for this step in the process? 5.) Assuming the PVDF-coated areas of the PVDF-Sn@Zn imprinted Zn anode are electrochemically inactive, do the authors have an estimate for the enhancement/reduction of the electrochemical surface area as compared to the geometric area of the normal foil? As the authors described in the introduction, surface area enhancement is one approach for reducing current density, but it is not clear if the SS mesh imprinting strategy leveraged in this work has a net enhancement or reduction in electrochemically active surface area compared to the baseline bare foil or imprinted/PVDF coated foil. Some discussion and basic calculations here would be helpful for interpreting the results. 6.) What are the rates shown in Figure S17? Please make sure these are appropriately normalized and the relevant mass (either electrode level or active material level) are given. 7.) What is the expected phase of the MnO2 cathode used in the full cell demonstration based on the XRD in Figure S15? 8.) The authors perform a corrosion analysis in Figure 3e. It would be useful to extract the associated exchange current densities using a Tafel analysis. This would also be interesting to compare to the values used for the finite element simulations. 9.) Do the authors have any electrochemical data comparing the imprinted Zn foil vs. regular Zn foil? At the beginning of the results section under "Gradient Electrode Design" the authors mention comparisons between the bare/3D Zn but only support these conclusions with modeling results (Fig. 1a/1b).
Reviewer #3 (Remarks to the Author): In this manuscript, Guan et al. manuscript reported an imprinted gradient Zn anode (noted as PVDF-Sn@Zn), where the PVDF-Sn@Zn gradient electrode showed a cycling of 1200 h at low current density/capacity of 1 mA cm−2/1 mAh cm−2 and 200 h at high current density/capacity of 10 mA cm−2/10 mAh cm−2. However, when coupled with the MnO2@C electrode as a full cell, the cycling performance of the PVDF-Sn@Zn was only slightly better compared with that of Sn@Zn. Moreover, the cycling stability of Sn@Zn||Sn@Zn pouch cell, PVDF@Zn ||PVDF@Zn pouch cell, MnO2@C||Sn@Zn pouch cell and MnO2@C|| PVDF@Zn pouch cell had not been provided. I cannot recommend this work for publication in Nat. Commun. at this stage. Here are my specific points: 1. In Line 143-144, the authors stated that "the thickness of the upper PVDF layer is about 2.4 um and the Sn layer is about 4 um". It is unclear that if these reported values of thickness were optimized parameters for the performance or not. Why did the authors set up the thickness of PVDF layer and Sn layer to be these values? I would suggest the authors to provide additional experimental results to optimize the setup.
2. In Line 215-216, the authors stated that "in Fig. 5d, PVDF-Sn@Zn||PVDF-Sn@Zn cell can maintain stable small voltage hysteresis (less than 15 mV) for over 1200 h …" However, the voltage hysteresis for PVDF-Sn@Zn in Fig. 5d varied significantly with the testing time. Why was that? In addition, I would suggest the authors to compare the voltage hysteresis after long cycling tests of this work with that of other works reported previously.
3. In Fig. 5e, the voltage hysteresis for PVDF-Sn@Zn turned smaller at 300-400 h compared with that at 0-300 h (there was a similar phenomenon in Fig. 5f). What were the possible reasons? Please discuss it.
4. In Fig. 5f, the whole testing duration was 200 h; nevertheless, the voltage hysteresis for PVDF-Sn@Zn only turned stable after 160 h. I would suggest the authors to extend the testing hours at 10 mA cm−2. Moreover, how about the voltage profiles of PVDF-Sn@Zn||PVDF-Sn@Zn cell at larger current densities/capacities? 5. In Fig. 6d, it seems that the capacity for PVDF-Sn@Zn was just slightly higher than that for Sn@Zn. Does this mean that the role of PVDF in PVDF-Sn@Zn is minor? Moreover, the long-term cycling stability after 630 cycles was missing. However, the authors stated in Line 250-251 "… initial capacity after 630 cycles, which is much better than the other three cells". This is less convincing.
6. In Fig. 6f-h, please provide the corresponding results of PVDF@Zn and Sn@Zn.
7. It would be helpful if the authors could provide theoretical calculations comparing PVDF-Sn@Zn with Zn foil, PVDF@Zn and Sn@Zn to validate the advantages of the PVDF-Sn@Zn gradient anode.

Response to reviewer's comments:
We thank the reviewers for the thorough evaluation of our manuscript. The valuable comments have enabled us to further improve the overall quality of the manuscript. The revisions are marked in red font color in the revised manuscript, and the point-to-point responses to the reviewers' comments are as follows.

Reviewer 1
The popularization of rechargeable zinc-ion batteries is seriously hindered by the zinc dendrite growth, electrolyte corrosion and side reactions on the Zn anode side. In this manuscript, the authors reported a simple imprinting method to construct imprinted gradient Zn anode (PVDF-Sn@Zn) that integrate gradient conductivity and hydrophilicity for long-term stable Zn-ion batteries. Systematic and comprehensive electrochemical and structural characterizations were applied to reveal the protection mechanism of double-gradient structure PVDF-Sn on Zn anode. Moreover, large-area (4*5 cm 2 ) MnO2@C//PVDF-Sn@Zn pouch cell assembled based on PVDF-Sn@Zn anode exhibits excellent performance indicators. Overall, this work proposed a novel and practical method to construct longterm stable Zn anode, but there are many defects and doubts in the manuscript that need to be revised. Therefore, I recommend this manuscript can be reconsidered after major revision. ____Thank you very much for the positive support and valuable comments. The manuscript has been carefully revised according to your suggestions. The detailed responses are as follows. ____We thank the reviewer for the valuable comment. Yes, the preparation of an artificial protective layer is an effective method to protect Zn metal from electrolyte corrosion and optimize Zn deposition behavior, thereby prolonging the cycle life of Zn anode. All the works mentioned by the reviewer belong to this strategy, and PVDF (Sn) is reported efficient. However, such reported planar artificial protective layers normally only worked effectively at low current densities/capacities (commonly less 7 than 5 mA (h) cm -2 ). At high current densities/capacities, the rapid growth of dendrites (hot spot effect) can be observed where the local electric field becomes large and protrusions appear on the electrode surface. Therefore, rational electrode design that reduces the electric field intensity on the electrode surface and optimizes the Zn 2+ flux is urgently needed to improve the stability of Zn anode. In addition, the growth of Zn dendrites is thermodynamically favorable, thus reducing the impact of dendrite growth on the cycle performance of Zn electrodes is also an important factor to be considered. With the above considerations, in this work, different from previously reported works, we report a facilely imprinted gradient Zn anode that well integrates gradient conductivity and hydrophilicity. The hydrophobic PVDF layer effectively prevents Zn metal from corrosion in the electrolyte, while the Sn layer with a high redox potential (Sn 2+ /Sn, -0.136 V vs SHE) inhibits the side reactions of Zn. More importantly, the gradient conductivity effectively induces electric field distribution, Zn 2+ ion flux and local current density toward the bottom of the microchannels, thus achieving desired bottom-up deposition behavior for Zn metal. Therefore, not only controllable and uniform Zn deposition is achieved, but also the possible short circuit from top dendrite growth is prevented. As a result, PVDF-Sn@Zn gradient electrode shows stable cycling of 1200 h at low current density/capacity of 1 mA cm −2 /1 mAh cm −2 and 200 h at high current density/capacity of 10 mA cm −2 /10 mAh cm −2 .
In all, although we used a common protective material of PVDF, the electrode design strategy and the mechanism of performance improvement is quite different. We have emphasized the novelty of our work in the revised manuscript.

Comments 2:
Zincophilicity and zincophobicity are important indicators for the construction of Zn anode protective layer, but they are not covered in this work.
____We appreciate the reviewer for the valuable comment. Yes, the rational design of electrodes with a zincophilic gradient is also an effective way to improve the stability of Zn anodes, similar conclusions have been confirmed in the design of lithium anodes. For example, Pu et al. 1 achieved a long-term stable lithium anode with lithophilic gradient by electrically passivating the top of a porous nickel scaffold and chemically activating the bottom of the scaffold. In this work, it is worth noting that Sn has good zincophilicity, which can effectively optimize the Zn deposition flatness and improve the stability of the Zn anode. 2,3 PVDF and Sn have obvious differences in both hydrophilicity and conductivity, which will greatly affect the nucleation and growth position of Zn. Since PVDF is a nonconductive material, Zn is not deposited on the surface of PVDF, but at the interface between PVDF 8 and Sn, where electron/ion transfer only occur at the surface of conductive Sn. Therefore, we do not emphasize the zincophilicity gradient considering that the deposition position is all at Sn layers. We have added more discussions on the zincophilicity and zincophobicity of Zn anode protective layer.

Comments 3:
The authors claimed that the PVDF-Sn@Zn electrode exhibits good flatness, but many obvious wrinkles can be observed from its optical photo (Supplementary Figure 1). ____We thank the reviewer for the valuable comment. The PVDF-Sn@Zn electrode was fabricated by a three-step fabrication process, including displacement reaction, imprinting process and PVDF coating. Both SEM images ( Fig. 2c-2e) and optical pictures (Fig. 3h) can prove that the PVDF-Sn@Zn electrode shows good flatness. However, when we tries on large-scale Zn foil, we used rolled Zn foil, the color differences caused by the small different Sn concentrations at different Zn surfaces would result in the obvious wrinkles in Supplementary Fig. 1. Also, the fast reaction between Sn 4+ and Zn would result in slight difference of the thickness of Sn in different area. We believe such structure color-induced wrinkles can be reduced using large-scale flat Zn foil and better control of the Sn 4+ reaction.

Comments 4:
The authors claimed that no additional peak can be detected in the XRD pattern of the PVDF-Sn@Zn gradient electrode after the immersing test. However, it is obvious that the XRD pattern of PVDF-Sn@Zn gradient electrode has several additional peaks at 15-30°.
____We thank the reviewer for the valuable comment. After careful evaluation, we do observe additional peaks in the PVDF-Sn@Zn gradient electrode after the immersing test. And we are sorry 9 about the wrong statement. We further re-evaluated the XRD results of the different electrodes after the immersing test and zoomed in the curves at 15-30°. As shown in Fig. R1, the PVDF-Sn@Zn gradient electrode shows a weak diffraction peak of Zn4SO4(OH)6·xH2O (ZSO), which proves to be effective in suppressing the formation of by-products. To better compare the corrosion resistance of the different electrodes, we compared the ratio of intensity of by-products to Zn metal (IZSO(002)/IZn(100)), where the values of Zn foil, PVDF@Zn, Sn@Zn and PVDF-Sn@Zn gradient electrodes are 8.2%, 2.6%, 2.7% and 1.8%, respectively (Fig. R1). The results further demonstrate the good corrosion resistance of the PVDF-Sn@Zn gradient electrode.
We have also added the discussions in Supplementary Information.

Comments 5:
The authors gave the Zn deposition model of PVDF-Sn@Zn gradient electrode in Figure 4, but did not prove its rationality. ____Thank you for the review's comment. The Zn deposition model in Figure 4 is intended to demonstrate the bottom-up Zn deposition behavior in the microchannels in the PVDF-Sn@Zn gradient electrode. Such deposition behavior can be clearly revealed by the ex-situ SEM tests, as shown in Fig. 4b-4e. It can be found that at a low deposition capacity of 5 mAh cm -2 , the deposited Zn is almost presented at the bottom of the microchannels, while no obvious deposition emerges on the top where PVDF exists, proving that the gradient design preferentially induces Zn to nucleate and grow at the bottom of the microchannels (Fig. 4c). As the deposition capacity increases, the microchannels are 10 gradually filled (Fig. 4d) and eventually a uniform and flat surface is developed when the deposition capacity reached 15 mAh cm -2 (Fig. 4e).

Comments 6:
The current density was not indicated in Supplementary Fig. 17. ____We thank the reviewer for the valuable comment. We apologize for neglecting the current densities in Supplementary Fig. 17. The current densities have been added, as shown in Fig. R2.

Comments 7:
Energy density is an important performance indicator of the pouch cell, and it is necessary for the authors to give. ____We thank the reviewer for the valuable comment. MnO2@C//PVDF-Sn@Zn pouch cell exhibits an energy density of 163.3 Wh kg -1 with a power density of 2286.1 W kg -1 at the current density of 2 A g -1 .
The energy density and power density of MnO2@C//PVDF-Sn@Zn pouch cell is calculated by the following equations: where E is the energy density, P is the power density, I is the discharging current, dt is the time differential, Δt is the discharging time, and m refers to the mass loading of MnO2.

Reviewer 2
In this manuscript the authors describe a four step series of chemical and mechanical processes for modifying a conventional Zn foil anode to address known challenges with this cell chemistry (poor plating/stripping reversibility, dendrite growth, cycling stability, utilization, and corrosion/side reactions). This series consists of the following steps: 1.) A simple displacement reaction is applied to introduce Sn to the surface of Zn foil.

2.)
The Sn@Zn foil is then imprinted with a stainless steel mesh to produce a microchannel surface pattern.
3.) A layer of PVDF is cast on the surface of the Sn@Zn and then dried. (https://www.sciencedirect.com/science/article/pii/S1385894721001832). Thus, while the individual approaches are not novel, it is the combination of these approaches that is the unique feature of this manuscript and would be of some interest to the Zn community.
In general, the work supports the conclusions, the methodology is sound, and there is enough detail provided in the methods for the work to be reproduced. This paper is suitable for publication in Nature Communications if the following questions and/or minor revisions can be addressed: 13 ____Thank you very much for the positive support to our manuscript and the valuable suggestions.
We have carefully revised our manuscript according to your comments, and the detailed answers are as follows:

Comments 1:
Are the anode images in Figure S11 shown in the charged (plated) or discharged (stripped) state? This should be included in the caption for context. ____We thank the reviewer for the valuable comment. Figure S11 shows the morphology of the PVDF-Sn@Zn gradient electrode after cycling. The cyclic process starts with plating and ends with stripping, so the images in Figure S11 are in the stripped state. We have added the context in the caption.

Comments 2:
Are the pouch cells cycled under any externally applied pressure? Was the impact of stack pressure explored?
____We thank the reviewer for the valuable suggestion. The cycling test of the pouch cells is performed under pressure (by pressing a 50 ml/1.8kg stainless steel reactor against the cell) to ensure better contact between cell components. We have also explored the cycling performance of pouch cells with/without pressure conditions. As shown in Fig. R3, pouch cell without pressure shows a rapid capacity drop, which can be attributed to poor contact between cell components and high ion/electron transfer resistance. In contrast, pouch cell under pressure exhibits good capacity retention after cycling. prepare PVDF-Sn@Zn gradient electrodes with 800-mesh SSM. However, during the rolling process, the 800-mesh SSM is easily peeled off from the Zn foil and cannot be maintained for the following coating procedure, therefore it could not be used to prepare gradient electrodes.  We also obtained a thicker Sn layer by soaking the Zn foil in SnCl4 solution for 10 min, as shown in To obtain the increased area of the microchannel structure compared to the planar one, we built Model 2 (Fig. R7b, front view), where the perimeter of cross-section A is π*28 (assuming that half of each SSM wire is imprinted into the Zn metal). Therefore, the area of half a cylinder is about π*28/2*l, by fixing length as a unit one. The area of a planar electrode under the same conditions is about 28*1.
Thus, it can be obtained that the active area of the PVDF-Sn@Zn electrode in the microchannels increases by about 57.1% (π/2-1). Accordingly, the ratio of the active area of the PVDF-Sn@Zn electrode can be calculated as 122.2% (77.8%*1.571), which means that the active area of the PVDF-Sn@Zn electrode is 22.2% higher than that of the planar electrode.
In addition, for electric field, it is ideal to assume that the entire electric field is concentrated in the microchannels. According to the electric field simulation, there is still a part of the electric field in the PVDF-coated areas, which can share the electric field intensity. The overall area of the electrode after imprinting is increased by 44.4% (77.8%*57.1%) compared with the planar electrode. Therefore, it is beneficial to reduce the electric field intensity of the whole electrode.
We have added the discussions in Supplementary Information.

Comments 6:
18 What are the rates shown in Figure S17? Please make sure these are appropriately normalized and the relevant mass (either electrode level or active material level) are given.
____We thank the reviewer for the valuable suggestion. We apologize for neglecting the current densities in Supplementary Fig. 17. The current densities have been added, ss shown in Fig. R2.

Comments 7:
What is the expected phase of the MnO2 cathode used in the full cell demonstration based on the XRD in Figure S15?
____We thank the reviewer for the valuable suggestion. The synthesis of MnO2 cathode in this work is referred to previous reported study with minor modification. 1-3 XRD pattern revealed that the obtained products are indexed to pure tetragonal δ-MnO2 phase (JCPDS 80-1098).

Comments 8:
The authors perform a corrosion analysis in Figure 3e. It would be useful to extract the associated exchange current densities using a Tafel analysis. This would also be interesting to compare to the values used for the finite element simulations.
____We thank the reviewer for the valuable suggestion. Figure 3e shows the corrosion resistance of the PVDF-Sn@Zn gradient electrode, obtaining an exchange current density of 1.26*10 -4 mA cm -2 .
The test was carried out in a 1 M Na2SO4 solution and the exchange current density represents the corrosion capability of the electrolyte on the electrodes. It is quite different from the exchange current density in the COMSOL simulations, where the value is related to the kinetics of Zn deposition and it is calculated using the following equation 1,2 : (3) here, is the total overpotential, and i0 is the exchange current density. As shown in Fig. R8, the PVDF-Sn@Zn gradient electrode exhibits an exchange current density (for Zn deposition) of 13.1 mA cm -2 , which is used in the finite element simulations. We have also added the discussions in Supplementary Information. We have carefully revised our manuscript according to your comments, and the detailed answers are as follows:

Comments 1:
In Based on the above discussion, it is favorable to achieve good cycling stability of the gradient electrode when the thickness of PVDF is 2.4 μm, and the thickness of the Sn layer is 4 μm.

Comments 2:
In Line 215-216, the authors stated that in Fig. 5d, PVDF-Sn@Zn||PVDF-Sn@Zn cell can maintain stable small voltage hysteresis (less than 15 mV for the first 600 cycles) for over 1200 h. However, the voltage hysteresis for PVDF-Sn@Zn in Fig. 5d varied significantly with the testing time. Why was that? In addition, I would suggest the authors to compare the voltage hysteresis after long cycling tests of this work with that of other works reported previously.
____We thank the reviewer for the valuable comment. Figure 5d shows  Table R1, the PVDF-Sn@Zn gradient electrode exhibits promising voltage hysteresis during long cycling.
We have also added the discussions in Supplementary Information.

Comments 4:
In Fig. 5f  We tested the cycling performance of the PVDF-Sn@Zn||PVDF-Sn@Zn cell at higher density/capacity (15 mA cm -2 /15 mAh cm -2 ). However, due to the high Zn deposition capacity, it is difficult to maintain the good Zn plating/stripping reversibility, and the PVDF-Sn@Zn||PVDF-Sn@Zn cell develops a short circuit after only short cycles (Fig. R12c).

Comments 5:
In Fig. 6d, it seems that the capacity for PVDF-Sn@Zn was just slightly higher than that for Sn@Zn.
Does this mean that the role of PVDF in PVDF-Sn@Zn is minor? Moreover, the long-term cycling stability after 630 cycles was missing. However, the authors stated in Line 250-251, initial capacity after 630 cycles, which is much better than the other three cells. This is less convincing.
____We thank the reviewer for the valuable suggestion. In this work, the role of PVDF in PVDF-Sn@Zn is mainly contributed to enhanced cycling stability. The higher capacity exhibited by the PVDF-Sn@Zn gradient electrode compared to the Sn@Zn electrode can be attributed to the microchannels induced by the imprinting process, which optimized Zn 2+ ion flux and increased specific surface area. More importantly, the top PVDF layer and the bottom Sn layer creates gradient conductivity and hydrophilicity in the electrode, which induces the preferential deposition of Zn in the microchannels. As a result, the PVDF-Sn@Zn gradient electrode can exhibit much better Zn plating/stripping kinetics and long-term cycling performance. 28 To further demonstrate the good cycle stability of the PVDF-Sn@Zn gradient electrode, we extended the cycle performance of the full cells to 700 cycles. As shown in Fig. R13, the MnO2@C//PVDF-Sn@Zn cell can retain 70.3% of its initial capacity in the 700th cycle, obviously better than the other three cells (MnO2@C//Sn@Zn-53.3%, MnO2@C//PVDF@Zn-43.4% and MnO2@C//Zn foil-failed). We have revised accordingly in the manuscript.

Fig. R13
Long-term cycling stability at a current density 2 A g -1 of different full cells.

Comments 6:
In Fig. 6f-h, please provide the corresponding results of PVDF@Zn and Sn@Zn.
____We thank the reviewer for the valuable comment. The cycling performance of the symmetric pouch cell, the CV curve and the cycling performance of the full pouch cell based on the PVDF@Zn and Sn@Zn electrode are shown in Fig. R14. As shown in Fig. R14a, the PVDF-Sn@Zn//PVDF-Sn@Zn pouch cell maintains a small voltage hysteresis and stable cycling for more than 500 h at a current density/capacity of 2 mA cm -2 (40 mA)/1 mAh cm -2 (20 mAh), better than Sn@Zn//Sn@Zn (231 h), PVDF@Zn//PVDF@Zn (63 h) and Zn foil//Zn foil (36 h) pouch cells. MnO2@C//PVDF-Sn@Zn pouch cell also exhibits superior reaction kinetics with lower voltage polarization than that for other three pouch cells (Fig. R14b). The stability test reveals that the MnO2@C//PVDF-Sn@Zn pouch cell can retain more than 65.5% of the initial capacity in the 250th cycle, which is also much better than the value for MnO2@C//Sn@Zn (49.3%), MnO2@C//PVDF@Zn (39.1%) and MnO2@C//Zn foil (32.3%) pouch cells (Fig. R14c). We have revised accordingly in the manuscript.

Comments 7:
It would be helpful if the authors could provide theoretical calculations comparing PVDF-Sn@Zn with Zn foil, PVDF@Zn and Sn@Zn to validate the advantages of the PVDF-Sn@Zn gradient anode.
____We thank the reviewer for the valuable suggestion. To reveal the advantages of the PVDF-Sn@Zn gradient anode over the other three controlled electrodes, theoretical calculations of the current density and Zn 2+ ion concentration distribution are studied in Fig. R15.
Current density distribution indicates that the roughness of Zn foil would result in concentrated current density on the bumps, which causes severe polarization (Fig. R15a). The non-conductivity of the PVDF layer can effectively reduce the current density on the electrode surface, but it is still uneven in these bumps (Fig. R15b). The Sn layer with good electrical conductivity also cannot solve the problem, with current density accumulated in bumps and resulting in uneven Zn deposition (Fig. R15c).
In comparison, the non-conductive PVDF layer on the top of the PVDF-Sn@Zn gradient electrode can effectively reduce the current density at the top surface of the electrode, thereby inducing the preferential deposition of Zn in the imprinted microchannels and improving the cycle performance (Fig. R15d).
The Zn 2+ ion concentration distribution shows that the Zn 2+ ion concentration on the Zn foil is low due to its unsatisfactory Zn affinity and high nucleation overpotential (Fig. R15e). The non-conductive 30 and hydrophobic PVDF will further reduce the Zn 2+ ion concentration on the surface of Zn metal (Fig.   R15f). The Sn layer can increase the Zn 2+ ion concentration of the Zn/electrolyte interface due to its good zincophilicity and hydrophilicity, thus can effectively improve the Zn deposition uniformity (Fig.   R15g). PVDF-Sn@Zn gradient electrode benefits from the advantages of the 3D structure, which can accelerate the diffusion of Zn 2+ ions and increase the Zn ion concentration of the electrode surface. In addition, the gradient design induces the Zn 2+ ion concentration incline to microchannels, resulting in uniform deposition of Zn in the microchannels (Fig. R15h).
We have also added the discussions in Supplementary Information.